1. Field of the Invention
The present invention relates to a gear shift control system of an automatic transmission installed in an automobile and, more particularly, to an automatic transmission gear shift control system in which, at the same time that a first frictional element of a plurality of frictional elements is coupled, a second frictional element is released or unlocked. The invention also relates to a method for performing such a gear shift control.
Typically, an automatic transmission installed in an automobile combines a torque converter and a transmission-gear mechanism and is provided with a plurality of frictional elements, such as brakes and clutches, which switch the torque transmission paths of the transmission-gear mechanism. The transmission also typically includes a hydraulic circuit which controls the application of hydraulic pressure for engagement to the frictional elements and the release of hydraulic pressure from them. Such an automatic transmission is designed to shift gears automatically according to driving conditions by conducting selective application of hydraulic pressure for engagement to the plurality of frictional elements which is performed by pre-designed operation of shift valves in the hydraulic circuit.
With regard to this type of automatic transmission, as disclosed, for example, in Japanese Unexamined Patent Publication No 63-186056, one of a plurality of frictional elements, i.e. a first frictional element, engages at the same time that another or second frictional element releases, thereby completing shifting between the specified gears as a result of transmitting torque from the second frictional element to the first frictional element.
In the above automatic transmission, one of a plurality of frictional elements, namely a first frictional element, engages at the same time that another, or second, frictional element releases in order to execute the specified gear shifting. It is important to timely manage the engagement action of the first frictional element and the release action of the second frictional element. In particular, when the timing is not proper relative to the input into the automatic transmission, that is, the output of the engine, what is called a "shift shock" occurs.
To understand the drawback of the prior art automatic transmissions, reference is made to FIG. 1. If the commencement of release of the second frictional element is earlier than the commencement of engagement of the first frictional element as indicated by a solid line, then torque transfer is conducted in a state where the engaging or locking force for these frictional elements is comparatively small, that is, in a state where a transmission mechanism is rather close to neutral, as shown by an arrow a. As a result, when an input torque from the engine is particularly large, both frictional elements become unable to withstand this input torque, and racing of the engine occurs. In contrast, if the commencement of the release action of the second frictional element is slower than the commencement of engagement of the first frictional element as indicated by a broken line, then torque transfer is conducted in a state where the engagement or locking force for these frictional elements is comparatively large, that is, in a state where the transmission mechanism is rather close to a double-lock condition, as shown by an arrow b. When the input from the engine is small, therefore, the output torque from the automatic transmission to the driving wheels drops, so that a shift shock occurs.